Water phantoms are well known in the art and are typically used for establishing the effect of ionizing radiations over the human body. Such a water phantom mainly comprises a water tank (with a volume of about 250 liters), and is equipped with driving unit which moves a radiation detector (e.g. a water-tight air ion chamber probe, a diode or an array of sensors) within the water tank volume into a plurality of measuring positions. The water tank is filled and emptied by means of a pump control mechanism from an external water reservoir.
A well known water phantom is, for example, the “Blue Phantom” manufactured by the assignee of this application, Ion Beam Applications S.A. Louvain-La-Neuve, Belgium. In such a water phantom, the detector may be a single sensor which is progressively positioned in a plurality of measurement positions until a full profile of the radiation beam has been obtained. The “Blue Phantom” may also be equipped with a linear array of detectors which can be moved in two dimensions in the water tank, in order to build a three dimensional map of the radiation field. One example of such an array of detectors is a linear diode array, available as the LDA-99 from Ion Beam Applications S.A. The LDA-99 comprises 99 diodes with 5 mm spacing.
According to known practice, measurements of radiation fields can be performed in two different configurations. In the first configuration known as TPR (tissue to phantom ratio) or SDD (source detector distance) configuration, the source-to-detector distance is fixed as the source-to-water-surface distance varies; and hence, the measurement depth varies during the measurement. The TPR or SDD configuration requires that the water phantom be equipped with a filling level sensor and a bi-directional pumping control allowing adjustment of the water level. In the second configuration known as DD (depth dose) or SSD (source to surface distance) configuration, the source-to-water-surface distance (i.e. the water level) is kept constant as the source-to-detector distance varies. Both configurations (TPR and SSD) are typical and complementary applications during radiotherapy unit commissioning and quality assurance (QA).
WO2007128087 A2, filed by Ion Beam Applications, S.A., describes a water phantom which mainly comprises: a water tank; means for varying the water level in the water tank; and an acquisition detector. According to the '087 PCT publication, the acquisition detector is a two dimensional detector comprising a plurality of sensors and capable of simultaneously measuring the dose in a plurality of points in an area. This acquisition detector is located beneath the water tank in a fixed position with respect to the water tank and opposite to the beam. Subsequent measurements are performed by varying each time the water level within the water tank, until the dose distribution in the entire volume of the water tank is obtained. The '087 publication describes the water phantom as capable of performing measurement in both TPR and SSD configurations.
Though the use of water phantoms has been an established standard since many years, the handling of these large scanning water phantoms is cumbersome and time consuming partly because of the long filling and emptying time of the water tank.
During the preparation of water phantom measurements, the water tank, the sensor moving mechanics, and the water surface have to be thoroughly aligned with respect to the radiation field. It is evident that in order to setup the water phantom accurately, the water surface has to be as calm as possible so that one can properly position the detector with respect to the water surface in order to get well aligned scans (typical accuracy required for the detector-to-water depth is about 0.5 mm). On the other hand, since the source-to-water-surface distance is an important measurement parameter, it is also important to have a calm water surface in order to setup the source-to-water-surface distance accurately. That is why conventional water phantoms, with typical water flow rates of the incoming water into the water tank of about 20 l/min, are subjected to water waves during the filling process. The presence of such water waves is actually a problem for the setup of the water phantom since they make the water surface very turbulent for quite a while even after filling the tank. Consequently, conventional water phantoms require additional time before one can start measurements waiting for the water within the tank to calm down.
This drawback is even aggravated when performing measurements in TPR configuration wherein the water level is changed during the measurement. In fact, conventional water phantoms require that the flow rate of the incoming water into the water tank is kept low since a high flow rate would create waves within the tank and make the water very turbulent thereby leading to noisy and inaccurate TPR measurements. It is therefore desirable to have a filling and emptying mechanism for the water tank that allows for higher water flow rates while at the same time minimizing the perturbing effects of water waves and turbulences.
The present invention aims to provide a water phantom that overcomes all above-discussed drawbacks of prior art.
More particularly, the present invention aims to provide a water phantom that allows performing a much faster setup as well as faster measurements with respect to prior art, especially in case of TPR configuration.